1. Field of the Invention
Finding the direction with regard to the true north is a necessity in everyday life, for example, in traveling, camping, sightseeing, driving, sailing, flying, exploration, land surveying, military activities, and the utilization of solar energy. Since the invention in the 11th century, the magnetic compass has been the most popular instrument for finding directions. It is well-known that the magnetic compass has a number of problems. The magnetic north is often grossly different from the true north. In the United States, the error (magnetic inclination) could be greater than ±20°, which also varies with location and time. In Canada, the error could be greater than ±45°. Furthermore, the magnetic compass is grossly affected by ferromagnetic materials in the neighborhood of the compass, for example, iron ore or steel pieces, such as vehicles and firearms.
The current invention discloses a novel compass which can automatically point to the true north based on the position of the sun, which is as easy to use as the magnetic compass, but much more reliable and accurate.
2. Description of the Prior Art
In 1834, W. A. Burt invented a solar compass which uses the position of the sun to determine the true north. Because of its reliability and accuracy, since the middle of the 19th century, the US government defined the solar compass as the standard for land surveying. The solar compass is also used in the military for finding directions in the battle field, for example, the Astro Compass MKII was used extensively in WWII, the Korean War, and the Vietnam War. Recent improvements of the solar compass were disclosed, for example, in U.S. Pat. No. 4,899,451 by Dandurand, and U.S. Pat. No. 5,424,178 by Steele et al., and U.S. Pat. No. 5,459,931 by Waltho. However, the operation of those solar compasses is very complicated, which requires the calculation of the local solar time versus the local standard time at the time of measurement, and requires elaborate manual adjustments. When a gnomon is used, the image is fussy, and the equation of time correction must be accounted for. Therefore, to date, the solar compass is used only by few experts.
The advance of GPS (Global Positioning System) greatly facilitates the measurement of the location (longitude, latitude, and altitude). However, the GPS signal does not contain direction information. To find the direction of a line, at least two GPS points are required. To achieve sufficient accuracy, a distance of hundreds of feet is necessary. The direction is computed from the difference of the individual positions. Although very accurate, its application is complicated and time-consuming.
In a previous patent application (Ser. No. 11/840,428, 08/17/2007 by Chengjun Julian Chen), the present inventor discloses a new type of optical device, the spherical omni-directional lens, which can focus the sun beam from all directions onto a sharp spot with a long depth of field, with an application to an accurate and easy-to-use solar compass. Although it is much more convenient than the earlier solar compasses, it still requires manual adjustments to find the true north.